Catalyst-Free Epoxy Vitrimers Based on Transesterification Internally Activated by an α–CF₃ Group

Abstract

Vitrimers are polymer networks in which associative exchange reactions can take place under specific conditions, thus conferring reprocessability to the insoluble materials. Recently, catalyst-free vitrimers have emerged as a new generation of vitrimers able to overcome potential leaching, ageing and sintering issues of catalysts and to ensure the preservation of the vitrimer properties after numerous reshaping processes. Here, a catalyst-free epoxy vitrimer featuring α-CF3-substituted ester functions is reported. First, a new tetra-acid precursor was synthesized via a catalyst-free thia-Michael addition. A catalyst-free ring-opening polymerization was then performed on two different di-epoxy monomers (DGEBA and BDGE) to obtain polymer networks composed of ester linkages. Curing was evaluated by rheology, DSC and FTIR monitoring of the polymerization kinetics. A gel content of over 70 % was measured after 24 h in THF. Finally, the accelerating effect of the α-CF3 group on the transesterification reactions was highlighted by stress relaxation experiments and analyzed computationally on a molecular model system. Reprocessability tests were carried out at 150 °C for 2 h. The mechanical and thermal properties of the reshaped materials were similar to those of the initial ones. This study demonstrates the potential of fluorinated groups as powerful internal activators for transesterification vitrimers.

Understanding the Reshaping of Fluorinated Polyester Vitrimers by Kinetic and DFT Studies of the Transesterification Reaction

“…Our group has recently reported polyester vitrimers featuring two fluorine atoms in α‐position the ester functions, which indeed can be reshaped under mild conditions without catalyst and thus confirmed the above‐mentioned F‐effect hypothesis [17] . Additional work has also revealed that the reshaping of transesterification vitrimers can be accelerated by the introduction of an α‐CF 3 group (fluorine atoms in β‐position), which was supported by DFT calculations on a model transesterification reaction [18] . Following those initial investigations, we have conducted more extensive DFT calculations in order to quantitatively assess the effect of neighbouring fluorinated groups on the transesterification reaction rate.…”

“…Following the recently reported acceleration for the reshaping process in transesterification vitrimers upon introduction of F atoms at the ester α [17] and β [18] positions, it was of interest to use the computational approach to understand the origin of this effect and to predict how to possibly further improve the transesterification rate.…”

“…A comparison of the activation barriers for the model degenerative exchange of the propionate esters CX 3 CH 2 COOCH 3 (X = H, F) with methanol has already been reported. [18] In the present contribution, we have expanded this investigation to systems having a variable number of F atoms at the ester α and β positions, R F COOCH 3 (R F = CH 3-n F n CH 2-m F m ; n = 0-3, m = 0-2), as well as on the alcohol (CH 3-n F n OH, n = 0-3).…”

“…[17] Additional work has also revealed that the reshaping of transesterification vitrimers can be accelerated by the introduction of an α-CF 3 group (fluorine atoms in β-position), which was supported by DFT calculations on a model transesterification reaction. [18] Following those initial investigations, we have conducted more extensive DFT calculations in order to quantitatively assess the effect of neighbouring fluorinated groups on the transesterification reaction rate. In addition, we have conducted experimental kinetic studies on model molecules to verify and validate the DFT predictions.…”

Design, Synthesis and Characterization of Vitrimers with Low Topology Freezing Transition Temperature

“…The viscosity of these polymeric materials gradually decreases with temperature as the rate of intermolecular transesterification increases following the Arrhenius law, which is a typical property of vitreous silica; hence, these polymers are called vitrimers [14]. This pioneering work on vitrimers and the subsequent reports have motivated many other research groups to design and synthesise various vitrimers using not only transesterification reactions [15][16][17][18] but also other dynamic covalent chemical processes, such as transaminations [19][20][21], transalkylations [22][23][24][25][26], siloxane-silanol exchange [27,28], olefin-metathesis exchange [1,29], disulfide exchange [30][31][32], and conjugate addition-elimination reactions [33]. The studies of such bond reorganization allows us to gain mechanistic insights [1] and specific applications such as shape memory behaviour [27].…”